A commercial telecommunications network operated by a service provider typically supports voice and/or data communications between various customer locations served by the network. An overall communications system may be subdivided into an access network and a core network, which may or may not be owned and operated by different service providers. Generally, customer devices communicatively couple to the access network which, in turn, connects to the core network. The access network includes what many people refer to as “the last mile,” that is, the connectivity from a customer location, such as an office building, to a point where a service provider has significant facilities, such as a metro hub or a “service edge” at the periphery of the core network. In contrast to the access network, the core network usually provides transport of large aggregate flows over long distances and handles the selective routing of each customer's voice and data traffic to other locations served by the network.
In order to provide desired communications services to a customer, one or more network elements must be provisioned for the service, and often one or more physical connections for carrying data signals must be established as well if they are not already in place. In the case of a new customer, a new physical connection is often to couple the customer location through to the access network and service edge. Part of the provisioning process would be to perform one or more tests to ensure that the proper connections were made and the equipment is capable of providing the required service. The information conduits, or links, that interconnect the network elements, must of necessity be set up through either active or passive components. The active components can be somewhat sophisticated, with the electronic and/or optical transmitters and receivers comprising customized integrated circuits, ASICs, FPGA's, etc. Because both active and passive components sometimes fail, it is an important function of the system to test the links and their associated components in order to ensure that they are functioning properly. One of the techniques used for testing these links is to perform a loopback test. Another test performed for trouble isolation is time domain reflectometry, or TDR.
Loopback is a well established test technique to test the transmitter and receiver logic as well as the physical link. Prior art loopback techniques essentially set the communications device up for full duplex operation where the transmitter and receiver operate concurrently and a link is established between the transmitter and the receiver by physically connecting the transmit data output to the receive data input. With the transmitter and receiver physically connected, a successful transmission can be verified by comparing the received data to the transmitted data. A significant drawback of prior art loopback techniques is that the link is typically taken out of service for the duration of the loopback test. No revenue generating customer traffic is carried during the test.
TDR is a technique whereby an electrical (or optical) signal is sent down a transmission path. The test signal will reflect off of impedance discontinuities in the transmission path, which may comprise open or short circuits. The reflected signal is then measured, and may be used to determine where a fault lies along the transmission path.
Previously, loopback and TDR testing were performed manually. To test a link, then isolate and correct problems, it is common for service providers to dispatch a truck to one or more sites to perform on-site diagnostic tests. Such testing and trouble shooting procedures are time consuming and expensive. This type of testing is often described as being performed at the physical layer, corresponding to Layer 1 of the Open Systems Interconnect (OSI) Reference Model, a well known concept in the industry.
There is a need in the art to test communications links, or perform trouble isolation testing, in an automated fashion, particularly in an access network where network elements and communication links may be scattered among different locations or may not be freely accessible to the service provider. Further, there is a need to perform link testing without taking the link out of service for the duration of the test.